Critical configurations have been established with enriched uranium in the form of squat 15.0-in. diameter cylinders and elongated 3.24-in. diameter cylinders. These cores were reflected by depleted uranium, polyethylene, graphite, and water; also, the squat cylinder was unreflected and reflected by beryllium of various thicknesses. Critical systems of plutonium were squat 6.0-in. diameter cylinders and elongated 2.25-in. diameter cylinders reflected by normal uranium, graphite, water, and in one case, polyethylene. Observed critical heights and diameters were corrected to correspond to standard enriched-uranium and plutonium densities and concentrations. These are tabulated along with effective extrapolation distances.